This invention is directed to the development of a planar-doped valley FET as an alternative to a high electron mobility transistor (HEMT) and as an improvement upon a delta-doped FET.
High electron mobility transistors consist of a heterostructure, within which a two dimensional electron gas plane is interposed. This two dimensional electron gas (2DEG) provides high electron mobility at low temperatures. The high transconductance associated with HEMT's also contributes to high gain. However, one disadvantage of these transistors is that electron mobility decreases at fields over 100 volts/cm, which results in decreased power gain at high biasing voltages. Another disadvantage is that the active channel within HEMT's includes a hetero-junction which is difficult to fabricate.
As an alternative to the hetero-structure, a delta-doped FET has been proposed which includes a homo-structure within which the 2DEG is placed. This FET includes delta-doped layer incorporated, via molecular beam epitaxy (MBE) techniques, into a gallium arsenide (GaAs) epitaxial layer. This delta-doped layer creates a potential well in which a 2DEG layer can exist. The impurity atoms remain localized within the 2DEG, which functions as the conduction channel. This also changes the energy-band diagram for the FET. More specifically, the resulting conduction band (C-band) is V-shaped as shown in FIG. 2a, wherein the C-band energy level drops below the Fermi level at the 2DEG layer. The V-shaped valley (also referred to hereinafter as the potential well) of a delta-doped FET (FIG. 2a), differs from a MESFET (FIG. 2b) or a HEMT (FIG. 2c), in that the delta-doped FET's C-band depends linearly (on the gate side of the potential well) upon the spatial coordinate z (the distance below the gate), whereas the MESFET and the HEMT exhibit a quadratic dependence upon z. This difference primarily depends upon the fact that MESFET's and HEMT's have constant doping concentrations below the gate (i.e. within the layer between the gate and 2DEG). In contrast, the GaAs layers on either side of the 2DEG of the delta-doped FET are undoped.
As a consequence the delta-doped FET offers advantages such as a high and tightly confined carrier concentration within the 2DEG, a high gate breakdown voltage, a desirable proximity of the 2DEG and the gate, and a high transconductance.
Delta-doped FET's achieve higher maximum carrier concentration within the 2DEG layer than HEMT's. The maximum donor concentration within a delta-doped FET is determined by the equation N.sup.2D.sub.D =(.epsilon./q)E.sub.av, wherein E.sub.av equals the avalanche breakdown field, .epsilon. is the permittivity of the semiconductor, and q is the charge. Assuming the E.sub.av equals approximately 6-7.times.10.sup.5 volts/cm, the maximum planar donor concentration is 5.times.10.sup.12 cm.sup.2. This concentration allows a carrier concentration which is significantly higher than the carrier concentration in single-channel HEMT's.
Delta-doped FETs exhibit a higher breakdown voltage than MESFET's or HEMT's, since the maximum electric field for a reverse bias delta FET is given by the equation E.sub.max =(1.div.d)(.phi..sub.B -V), wherein .phi..sub.B represents the Shottky barrier, d represents the depletion width of the delta FET, and V represents the gate-source bias voltage being applied. This equation differs from the conventional MESFET, since the maximum electric field for a conventional MESFET is given by the equation E.sub.max =(2.div.d) (.phi..sub.B -V). Thus, when the electrical field is at a maximum E.sub.max, the maximum breakdown voltage V within the delta FET will be higher than that of the MESFET having the same transconductance. HEMTs designed with the same transconductance as a MESFET, and having uniform doping in the AlGaAs layer between the gate and heterojunction, will have the same breakdown voltage as a MESFET. Typically, however, HEMTs are designed for higher transconductance than can be obtained in MESFETs, and consequently have a lower breakdown voltage.
Concerning the electron mobility, in a delta-doped FET using gallium arsenide as the homo-structure, electrons remain close to their parent donor impurities in real space due to electrostatic attraction. Therefore, delta-doped gallium arsenide layers have lower mobility than undoped gallium arsenide layers. The delta-doped gallium arsenide layers commonly have mobilities at approximately 3000 cm.sup.2 /v.sec, with a donor concentration of 2.times.10.sup.12 to 6.times.10.sup.12 cm.sup.-2.
The parasitic resistance within a FET between the source and gate, and gate and drain, depend on the low field mobility (i.e. as mobility decreases, parasitic resistance increases), thus high mobility is desirable. The HEMT, for example, has very low parasitic resistances due to the high carrier mobility; whereas a delta-doped FET has higher parasitic resistances due to the lower mobility. In order to reduce the parasitic resistances within a delta-doped FET, the gates are self aligned or a recessed gate structure is used. Thus, the adverse effects of parasitic resistance can be minimized.